PURPOSE: To surely trace a desired data signal by always storing the data signal as the trace object in a storage means by a storage pulse for storage of the data signal and using the same address for overwrite in the case of a data signal other than the desired data signal and counting up the address in the case of the desired data signal.

CONSTITUTION: Plural data signals 11 including the data signal as the trace object and a data signal 13 which specifies the data signal as the trace object out of these data signals 11 are collated with each other by a comparing and collating circuit 1. A counter 2 is counted by a collation coincidence pulse 14 outputted at the time of the coincidence between these data signals 11 and 13 to determine a data storage address 15. The data signal 11 coinciding with the data signal 13 is stored in a RAM 3 in accordance with the data storage address 15 by a storage pulse 12 which satisfies the pulse width standard for data storage.